Rearchitect platform memory map parsing to make it less
Open Firmware-centric:
- Keep the static list of regions in platform.c instead of ofw_machdep.c
- Move various merging and sorting operations to platform.c as well
- Move apple_hacks code out of ofw_machdep.c and into platform_powermac.c,
  where it belongs
- Move CHRP-specific dynamic-reconfiguration memory parsing into
  platform_chrp.c instead of pretending it is shared code
No actual hardware supported by FreeBSD requires this SPRG save/restore
paranoia, so kill it. In particular, changes to SPRG0 are dangerous, since
that is where the PCPU pointer is kept.